Austenitic steel alloy



3,169,858 AUSTENKTKQ STEEL ALLGY Gerald B. Heydt, Reading, and Ciyde Raymond Whitney,

Wyomissing, Pa, assignors to The Carpenter Steel Company, Reading, Pa, a corporation of New Jersey No Drawing. Filed May 24, 1062, Ser. No. 197,290 Ciairns. (6!. 75-124) This invention relates to an austenitic iron base alloy which is age hardenable and more particularly to such an alloy having high strength at elevated temperatures.

This application in part is a continuation'of our copending application filed on January 23, 1961, Serial No. 83,895, and assigned to the assignee of the present application.

Hitherto, for use where high strength at elevated temperature had been required austenitic iron base alloys were provided having a composition'bal'anced so as to respond to an aging or precipitation hardening treatment. More particularly such alloys were balanced so that upon aging they underwent a precipitation hardening and strengthening mechanism such that a precipitate was formed having a lattice structure the sameflas that of the alloy matrix in which it is formed and also having a lattice parameter which is close to but yet constitutes a misfit with the lattice parameter of the matrix. 7

The present invention is particularly concerned with improving those iron base-austenitic precipitation hardening alloys which harden during aging by a mechanism involving the formation of a face centered cubic precipitate known as gamma prime. The usefulness of such alloys has been limited by the fact that the gamma prime precipitate transformed during use at elevated temperature after a relatively short time so that the composition lost its strength. This transformation results from the fact that the gamma prime, face centered cubic structure formed in the original aging treatment underwent a further transformation to a hexagonal close packed structure, known as eta phase. The extent of the transformation depends upon the duration of the exposure to temperatures between about 1100 F. to 1700 F. and the degree of stress to which the part formed of the alloy is subjected.

In the absence. of stress at about 1400 -F., the transformation of the face centered cubic to the hexagonal close packed structure i apparent in as little as sixteen hours and after about one hundred hours the transformation has progressed to such an extent that the part formed of the alloy has insufficient strength for its intended use. The transformation of the face centered cubic gamma prime phase to the hexagonal close packed structure proceeds more rapidly at any given temperature between about 1100 F. and 1700 F. as the part is subjected to greater stress. Thus, in service where the parts formed of such alloys are subjected to stress at elevated temperature, they have been found to have an objectionably short useful life I While some measure of success has been achieved heretofore in providing such alloys having good strength at elevated temperatures for period of about one hundred hours as represented by stress rupture tests, such alloys have left much to be desired insofar as their stress rupture ductility was concerned. For example, such alloys were considered to be unduly prone to failure when subjected to stress at high temperature after having been scratched V or notched.

' United States Patent 0 resultsin the formation of unsound areas in forged billets 3,160,858 Patented Feb. 16, 1965 suited for use in forming such parts as wheels and rings for use in jet engines. Thus, the alloy of the present invention is not only characterized by high strength and stress rupture ductility at elevated temperatures but also is characterized by good resistance to oxidation and corrosion at such temperatures.

A more specific object of the present invention is to provide such an alloy which may be readily melted and cast into ingots that may be economically hot worked and formed into parts.

We have discovered that the foregoing objects can be achieved with an austenitic steel having the following analysis in percent by weight within the tolerance of good commercial melting practices:

the balance being principally all iron except for incidental impurities such as phosphorous, sulfur, nitnogen,-oxygen and the like, the ratio of the titanium content to the aluminum content being at least equal to about 2.

The proper balanceof the alloying elements of this composition is highly critical and must be carefully maintained if sound steel having the required properties at temperatures as high as 1500" R is to be obtained suitable for such use as in the fabrication of jet engine parts.

In this alloy, the maximum carbon content must not exceed about .10% and is preferably maintained at a much lower level below about .05% maximum. Vaniadium is included in this composition in amounts ranging from about .10% to 1% primarily to tie up the carbon and prevent the formation of a titanium carbide. When the carbon content is maintained below .05 then only about .15% to 3% vanadium is required for this purpose. Manganese may be present in amounts up to about 2% but is preferably limited to no more than .25

Silicon when present in more than very small amounts formed from relatively large commercifl ingots because A alloyis strengiheued. Whilefrom 20% to 3.0% nickel is of the formation of anickel-titanium-silicide segregate believedto be Ni Ti Si identified as G phase. Therefore, silicon is limited to no more than about .25 %'-as is brought out in said application Serial No. 83,895. Preferably, the silicon content is maintained below ,15% in order to ensure obtaining the soundest steel.

Small additions of boron work to improve the rupture strength of our alloy at elevated temperatures. For this purpose, from about 003% to .025 %bor on provides good results. When present in amounts above .025 %,iboron tends to form borides, particularly with nickel, andthis has a detrimental effect up on the forgeability and mechanical properties of the alloy. Preferably, lower amounts below 013% boron are'utilizedto provide optimum re chromium provides the requireddegree. ofoxidation and corrosion resistance; Because chromium is 'a relatively strong ferrite former, preferably about 15% to17% chromium is utilized in this composition. Nickelnot only tends'to favor the formation of austenite but also takes part in the aging mechanism by WlliCll'thfi present encase desirable, we, preferably utilize from about 25 to 27% nickel to ensure a fully'austenitic alloy and at the same 7 time provide sufiicient nickel for. the precipitation and hardening reaction.

, Molybdenum and/ or tungsten are included because they 5 improve the high temperature strength of the alloy. For

this purpose, molybdenum andtungsten are generally equivalent and'either or both may be included to provide a total addition ranging from about 1% to 3%. Best results 'are'a-chieved when molybdenum is utilized alone and prefenablyin an -amount ranging from about'LZS to Titanium largely contributes to the-rupture strength and the tensile strength of the alloy and for this purpose about 2.75 to 3.75 %titaniu-m is utilized. When titanium is present in an amount less than about 2.75 both the high room temperature tensile strength and the high I rupture strength at elevated temperatures characteristic 1 of our alloy are not attainable. ,Larger amounts of'titaniu-rn than about'3.75% tend to impair the hot workability ofthe composition. Titanium, in the absence of at least I 'about .75 aluminum, reacts with nickel during aging to provide a gamma prime phase identified as Ni Ti which ,has less stability than desired against transformation to the undesired eta phase. I

j We have'unexpectedlydiscoyered that when aluminum is present in our alloy in amountsvarying from about .75 to 1.75 the more stable gamma prime phase is formed on aging identified as Ni (TiAl), most consistent results being attained whenal-uminum is present in an amount varying from 1% to 1.5%. Our experiments j have shown thatthe improved stress rupture ductility and improved strength at elevatedtempenatures as high as 1500 F. is brought about by the presence in our alloy of must not be so greatias to cause weakening rather than strengthening of the alloy. We have found that when'the aluminum content is too closeto,eq-ual to, or gre-aterthan the titanium content, the resulting misfit between the lattice structures is excessive and results in a composition having low strength athigh temperature. Best results havebeen achieved when at any level f titanium within the limits st-ated, the ratio of the percent-by weight con tentof titanium to that of aluminum is at least'equalto f about 2.5 When the value of this ratio is decreased below about 2.5 .by increasing the'amount of the-aluminum. in relation tothe titariunn the high temperature strength of. j our alloyisafiected and when the ratio is less than about 2, the improved high tempenature strength of our alloy 1 is not obtained- Thus, the alloy whichwe'have produced withprefer-red I resultshas the following composition in percent by weight within' the' tolerances of good commercial melting prac being at least 2.5an'd' the balance being principally all iron except for incidentalimpurities. a p

Our alloy is readily prepared'and worked in accordance with good standard commercial practices. No special heat 7 treatment is required. ,Solution treatment at about 1800" F. to 1900" 'F.,for about one to two hours followed by oil or air. quenching and an aging or precipitation'harden ing treatment atabout .1300? F. to 1500 F. for about sixteen hours-provides good results. 1 V D As a specific example of our'alloy an ingot was melted and cast containing 021% carbon,,21% manganese, 21%

silicon, 14.40% chromium, 26.16% nickel, 1.12% molybdenum, 28% vanadium, 1.34% aluminum, 3.31%titanium, .010% boron and the balance all iron except for incidentalimpuritie's. The ingot washot worked to 'half inch bars, the-n solutiontreated and aged. in this instance the solution treatmentwas' carried out att1900 F. for

1 two hours followed by quenching. A hardness of Rockwell C 34 was obtained afteraging for sixteen'hoursat 1400 F. followed by cooling in'air as comparedto an as-solution-treated and quenched hardness-offRockwell B 76. When a specimen of this alloy was aged for'four hours at 1500 F. it showed ahardness of Rockwell C 33/34 and when agedfor twenty hours at that temperature a hardness of Rockwell C 32 was still obtained.

The agedbars were machined to form standardcoin binationsrnooth-notch.stress rupture specimens having a smooth section .195 inch in diameter and a gauge length of .780 inch merging with a thicker section .275 inchinl diameter in which a circular notch was formed having a .195 inch diameter at the root of the notch, a root radius. of .005 inch and a notchfangleoffiofi' At 1300 F., a stress load oreznoo p.'s.i. was sustained for eighty hours before rupture with a 17% elongation and 25% reduction in area. The failure. occurred in the smooth portion of the test specimen and not at the notch, indicating that the alloy has good stress ruptureductility- On the other hand, tests carried out on similarly pre- 'pared specimens but not containing'titanium and alurni num'in the stated ratio showed significantlylower att'ainable as-age'd' hardness as well as indications. of overaging when aged for sixteen hours at 14007 F; While such specimens showed good stress rupture ductility, they were characterized by poor. stress rupture strength. For example, analloy having essentiallythe analysis of the specific example given above but containing 2.38% alumi num' and 1.93% titanium was ruptured in the smooth area of the specimen after only seven hours when subjected to a stress of 62,000 psi; .at 1300 with a 27% elongation and32% reduction in area. For further I comparison, it may als'o'be noted that the last mentioned 'alloy when aged at 1400 F. for sixteen hours had shard ness of only RockwellC 28' and when aged for four.

hours at 1500" F. was over-aged to such an extent'that it had a hardness of only Rockwell B 97. a i

. The terms andexpress ions which have been employed 1 are used as terms of description and notof limitation,

and thereis no intention, in the use of such terms and expressions of excluding any. "equivalents of the features shown and described'or portions-thereoflbut it is recog nized that various modifications'are possible within the;

scopeof the invention claimed; I We claim: r

or fupto'l10,%- carbon, up to 2% manganese, upto 25%. silicon, 14%to 20% chromium, 20% to 3.0% nickel,'1%

. 103% molybdenum, 2 75% to 3.75% titanium, 15% f to 1,75%.alumin'um, .10% to 1.0% vanadium, .oom s;

.025 boron, theititanium content .beingsuificiently p p v H I greater thanthealiuninuni content to limit the extent of the ratio of the titanium'content to the aluminum content the misfit between the lattice parameter of the gamma" 7 7 1. An. age hardening austenitic' iron base alloy which in its hardened condition has good strength and ductility 'at temperatures uptbabjove 15009 R, which is essentially free of a silicon bearing segregate phase and which within ,7 the tolerances of good melting practice consists "essentially prime phase and that of the alloy matrix when aged, and the remainder essentially iron.

2. An age hardening austenitic iron base alloy which in its hardened condition has good strength and ductility at temperatures up to about 1500 F., which is essentially free of a silicon-bearing segregate phase and which within the tolerances of good melting practice consists essentially of up to .10% carbon, up to 2% manganese, up to .25% silicon, 14% to 20% chromium, 20% to 30% nickel, 1% to 3% molybdenum, 2.75 to 3.75% titanium, .75% to 1.75% aluminum, .10% to 1.0% vanadium, 003% to .025% boron, the ratio of the titanium content to the aluminum content being about equal to at least 2, and the remainder essenially iron.

3. An age hardening austenitic iron base alloy which in its hardened condition has good strength and ductility at temperatures up to about 1500" F., which is essentially free of a silicon-bearing segregate phase and which within the tolerances of good melting practice consists essentially of up to .10% carbon, up to 2% manganese, up to .25 silicon, 14% to 20% chromium, 20% to 30% nickel, 1% to 3% molybdenum, 2.75% to 3.75% titanium, .75% to 1.75 aluminum, .10% to 1.0% vanadium, .003% to .025% boron, the ratio of the titanium content to the aluminum content being about equal to at least 2.5, and the remainder essentially iron.

4. An age hardening austenitic iron base alloy which in its hardened condition has good strength and ductility at temperatures up to about 1000 E, which is essentially free of a silicon-bearing segregate phase and which within the tolerances of good melting practice consists essentially of up to .05 carbon, up to 25% manganese, up to .15 silicon, 15% to 17% chromium, 25% to 27% nickel, 1.25% to 1.75 molybdenum, 3% to 3.5% titanium, 1% to 1.5% aluminum, .15% to 30% vanadium, .003% to .013% boron, the ratio of the titanium content to the aluminum content being about equal to at least 2, and the balance essentially iron.

5. An age hardening austenitic iron base alloy which in its hardened condition has good strength and ductility at temperatures up to about 1500 F., which is essentially free of a silicon-bearing segregate phase and which within the tolerances of good melting practice consists essentially of up to .05% carbon, up to .25 manganese, up to .15% silicon, 15% to 17% chromium, 25% to 27% nickel, 1.25% to 1.75 molybdenum, 3% to 3.5% titanium, 1% to 1.5 aluminum, .15 to .30% vanadium, .003% to .013% boron, the ratio of the titanium content to the aluminum content being about equal to at least 2.5, and the balance essentially iron.

References Cited in the file of this patent UNITED STATES PATENTS 2,750,283 Loveless June 12, 1956 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,169,858 February 16, 1965 I Gerald B., Hey dt et al.,

It is hereby certified'that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected belo' In the grant, line 1, for "Gerald B. Hey-st" read Gerald B. Heydt column 5, line 14, for "essenially" read essentially column 6, line 2, for "1000 P." read Signed and sealed this 17th day of August 1965.

(SEAL) Altest:

ERNEST W. SWIDER EDWARD J. BRENNER Attesting Officer Commissioner of Patents 

2. AN AGE HARDENING AUSTENITIC IRON BASE ALLOY WHICH IN ITS HARDENED CONDITION HAS GOOD STRENGTH AND DUCTILITY AT TEMPERATURES UP TO ABOUT 1500*F., WHICH IS ESSENTIALLY FREE OF A SILICON-BEARING SEGREGATE PHASE AND WHICH WITHIN THE TOLERANCES OF GOOD MELTING PRACTICE CONSISTS ESSENTIALLY OF UP TO .10% CARBON, UP TO 2% MANGANESEN, UP TO .25% SILICON, 14% TO 20% CHROMIUM, 20% TO 30% NICKEL, 1% TO 3% MOLYBDENUM, 2.75% TO 3.75% TITANIUM, .75% TO 1.75% ALUMINUM, .10% TO 1.0% VANADIUM, .003% TO .025% BORON, THE RATIO OF THE TITANIUM CONTENT TO THE ALUMINUM CONTENT BEING ABOUT EQUAL TO AT LEAST 2, AND THE REMAINDER ESSENTIALLY IRON. 